The present invention relates generally to AC induction machines and, more particularly, to a system and method for determining rotor speed of an AC induction machine.
Electric motors consume a large percentage of generated electricity capacity. Many applications for this “workhorse” of industry are fan and pump industrial applications. For example, in a typical integrated paper mill, low voltage and medium voltage motors may comprise nearly 70% of all driven electrical loads. Due to the prevalence of these motors in industry, it is paramount that the electric motors be operated reliably and efficiently. Motor design parameters and performance parameters are often required by motor management systems to optimize the control and operations of electric motors. Similarly, motor status monitoring enables the electric motors to operate reliably. Many motor status monitoring techniques also look for certain motor design parameters and performance parameters.
One such motor performance parameter that is helpful in optimizing the control and operations of electric motors is rotor or motor speed. There are many different techniques for estimating motor speed, including complex techniques that are highly accurate but unreliable (i.e., not useful under all conditions) and simplistic techniques that are more reliable but less accurate. Two such motor speed estimation methods, respectively, are (1) motor equivalent models or complex digital signal processing techniques, such as Fast Fourier Transform (FFT) or other frequency-domain signal processing operations, or (2) a technique that implements a linear speed-load curve derived from rated motor speed (RPM) and synchronous speed (RPM). However, each of these techniques has limitations regarding the availability of implemented and/or limitations regarding the accuracy of the motor speed estimation.
With respect to implementing of FFT based speed estimation techniques, it is recognized that for low-end motor control or monitoring products, these techniques often may not be implemented because of hardware and software restrictions. Additionally, it is recognized that such techniques may not be reliable. That is, the accuracy is relatively high when a signal contains enough speed related information; however, when such information is not sufficient, the method can give inaccurate results.
With respect to linear speed estimation techniques, it is recognized that implementation may be limited to motors operating under rated conditions (rated voltage and rated frequency). However, for motors that are operating under rated conditions, such as inverter-fed motors, such motor speed estimation often cannot be used since the rated RPM in the nameplate is only valid for rated motor operations (e.g., at a rated voltage and a rated frequency). Additionally, even for linear speed estimation techniques that can be implemented with inverter-fed motors, it is recognized that errors in the linear motor speed may be present due to error in the rated speed from the name plate information of the motor and non-linear load-speed characteristics of the motor. While such errors may be small (less than 4%), it is still desirable to compensate for such errors in order to derive a more accurate motor speed estimation.
It would therefore be desirable to design a system and method for determining speed of an AC induction machine that is not dependent on set load, voltage, and frequency conditions, so as to enable the improved management and status monitoring. It would further be desirable for such a system and method to provide accurate estimation of the speed in a reliable fashion, regardless of the exact operating conditions of the AC induction machine.